This invention relates generally to a non-invasive system for monitoring the arterial oxygen saturation and more particularly to a method of detecting an irregular state in a non-invasive pulse oximeter system and to a method of suppressing a pulse alarm caused by a non-physiological event in a non-invasive pulse oximeter system for measuring the arterial hemoglobin oxygen saturation.
Nowadays, the hemoglobin oxygen saturation of arterial blood is often measured with a non-invasive technique, which is called pulse oximetry. Pulse oximeters measure the arterial oxygen saturation of hemoglobin using two different monochromatic light sources, which are typically formed by LEDs, one of them emitting light in the red wavelength range of about 645 nm, the other one emitting light in the infrared wavelength range of 940 nm. The light emitted by both LEDs is transmitted through a predetermined area of the patient's body. Typically, pulse oximeter systems utilize an oxygen saturation sensing probe which is arranged to be secured to the patient's finger. Usually, the probe has the form of a clip including both light emitting diodes and a light detector. The probe is arranged such that the light of both light emitting diodes having passed the predetermined area of the patient's body is received by a single light detector. As it is known in the art of pulse oximetry, the light of both light sources is attenuated by static and dynamic absorbers on its path through the patient's body to the light detector. The arterial blood whose quantity varies with the time synchronously with the patient's heartbeat represents the only dynamic absorber during the pulse period. All other absorbers, such as skin, tissue or bone, are not time-variant. As mentioned earlier, the light detector, which may have the form of a photodiode, receives the modulated light intensities of each wavelength. Then, these signals are amplified, low pass filtered, converted from analog to digital and further processed in a microprocessor system. A pulse finding algorithm analyses the received signals, which are so-called spectrophotometric signals for identifying the pulses and for determining the pulse. After identifying the pulse period, the microprocessor system determines the diastolic and systolic values of the spectrophotometric signals and derives therefrom the so-called relative absorption ratios. Subsequently, the microprocessor system computes the arterial oxygen saturation from the relative absorption ratio using calibration data and so-called extinction coefficients from the absorption spectrum of hemoglobin and oxyhemoglobin at the appropriate wavelengths. The pulse finding algorithm of the pulse oximeter will generate an alarm if no pulsation is detectable on the spectrophotometric signals.
Typically, non-invasive systems for monitoring the arterial oxygen saturation are also used for monitoring the patient's blood pressure. The blood pressure is monitored by utilizing an inflatable blood pressure cuff which can be wrapped about the patient's limb. The monitor determines the patient's systolic and diastolic blood pressure. In some specific medical applications, it is not possible to measure the blood pressure on one arm of the patient and to detect the oxygen saturation with a probe secured to the patient's finger of the other arm. Thus, if both the pressure cuff and the oxygen saturation probe are to be used on the same arm, a measurement of the blood pressure by, for example, inflating the cuff and conducting oscillometric measurements, will necessarily reduce the flow of blood and thus influence the oxygen saturation. Then, the above-described pulse finding algorithm system of the pulse oximeter will generate a "non-pulsatile" alarm. As will be appreciated by those skilled in the art of pulse oximetry, this alarm is not caused by physiological events. Rather, it is caused by the measuring method. Similarly, the inflation of the cuff influences the oxygen saturation. When the measure of oxygen saturation falls below a predetermined threshold, the so-called SaO2-alarm is generated. Further, the inflation of the cuff may influence the measured pulse rate so that same is no longer between predetermined tolerance thresholds which may also cause the generation of an alarm.
U.S. Pat. No. 4,776,339 discloses a patient monitor for measuring The non-invasive blood pressure of the patient and for monitoring the oxygen saturation of the patient's blood. The system comprises an inflatable cuff for disposition about the patient's arm to provide the blood pressure signal and a probe for securement on the finger of the patient to provide the oxygen saturation signal. The system also comprises an alarm circuit to provide a blood pressure alarm signal or an oxygen saturation signal in the event that the actual values of these signals deviate from predetermined acceptable values. A control circuit responds to the inflation of the cuff to feed a signal to the alarm circuit for disabling the oxygen saturation alarm if the cuff is inflated. Although this prior art system overcomes some of the above-mentioned problems of standard pulse oximetry systems, the field of application of the system is somewhat restricted as the alarm circuit requires an input signal indicative of the inflation of the cuff so as to disable the oxygen saturation alarm if the cuff is inflated. On the one hand, the input signal is not available in all systems. On the other hand, the technique also suffers from the problem that it disables more alarms than necessary. For example, dramatical changes in the oxygen saturation may be physiological and not caused by the inflation of the cuff. Therefore, any abrupt changes in the oxygen saturation arising during the inflated condition of the cuff will not be detected by this prior art system.
These problems encountered with the technique described in U.S. Pat. No. 4,776,339 were also recognized by the authors of U.S. Pat. No. 5,253,645 (see column 1, lines 32 to 56 thereof).
However, U.S. Pat. No. 5,253,645 discloses yet another approach for reducing the number of "false" alarms without suppressing relevant alarms. In accordance with the system described in U.S. Pat. No. 5,252,645, the amplitude of the oximeter sensor is compared with a threshold. A signal indicative of an oximeter sensor signal falling below the threshold is fed to a first timer. Further, a blood pressure signal is fed to a second timer. The output signals of both timers are logically combined for generating the audible alarm of the system. An additional visional alarm is created on the mere basis of the output signal of the first mentioned timer. To summarize, this prior art system requires a blood pressure signal and an oximeter sensor signal and derives an alarm by logically combining timer signals derived from these input signals.
In contrast to the above technique, it is the goal of the present invention to analyze an irregular state on the mere basis of an analysis of the spectrophotometric signals.